The present invention relates to a method of controlling the placement of a cross-linked polymer gel in a subterranean formation for reducing permeability and improving sweep control. More specifically, it is concerned with the sequential injection of a slug of a cross-linkable polymer, a cross-linking agent and sequestering agent to achieve deeper polymer placement.
For many decades, the oil and gas industry has recognized the desirability, especially in enhanced oil recovery operations, of being able to selectively control the permeability of hydrocarbon bearing formations in order to reduce unwanted water production and to optimize oil and gas production by reducing the "fingering" of enhanced oil recovery fluids through porous reservoir regions. Thus, concepts such as permeability control, formation plugging, selective fluid placement, mobility control and the like are an integral part and significant feature in many aspects of oil and gas production processes including waterflooding, miscellar flooding and other related processes. In particular, the oil and gas industry has generically suggested and commercially used a variety of water soluble polymer-forming reactants which after being injected into the water bearing portion of the formation are polymerized or gelled such as to reduce the permeability of the previous water producing region. One specific technique involves the placement in the formation of a cross-linkable polymer solution which is followed by a cross-linking agent. In actual commercial implementation, the most frequently suggested polymers are polyacrylamides, polysaccharides, cellulosic polymers and lignosulfonates. Similarly, the most frequently used cross-linking agents are aluminum in the 3+ valence state and the dichromate species.
The process which is of particular relevance to the present invention involves the sequential cyclic injection scheme of introducing an aqueous cross-linkable polymer solution into the subterranean formation followed by a slug of a cross-linking agent made up of a multivalent cation accompanied by a sequestering anion. In U.S. Pat. No. 3,762,476, a process for reducing the quantity of water recovered from a subterranean formation involving the in situ cross-linking of a partially hydrolyzed polyacrylamide with aluminum citrate is disclosed. The claimed process involves the injection of the aqueous polymer solution followed by a slug of complexing ionic solution of multivalent cations and retarding anions capable of gelling the polymer solutions, and then the injection of brine, followed by a second injection of the polymer solution. The brine serves to prevent premature polymer cross-linking in the injection lines prior to actual injection into the formation. In U.S. Pat. No. 3,833,061, the polymer/cross-linking agent/polymer injection sequence is improved by preflushing the formation with a solution containing an oxidizing agent to remove hydrocarbons from the surface of the formation prior to in situ gelation of the polymer. In U.S. Pat. No. 3,926,258, a single slug injection scheme is proposed wherein the cross-linkable polymer is placed in solution with a multivalent cation cross-linking agent at a valence state above the lower valence state which promotes cross-linking, plus a complexing agent and a reducing agent, thus achieving a delayed gelation and extended gel time for deeper formation penetration. In U.S. Pat. No. 3,981,363, the basic two-step sequential injection of polymer/cross-linking agent is modified in that the polymer is partially cross-linked prior to injection.
Although these processes have been commercially implemented and have met with at least partial success, a still-unresolved problem, prior to the present invention, involved the tendency for excessive polymer retention at the face of the formation in the wellbore where injection occurs, particularly after repeated injection cycles. This excessive polymer retenion led to severely limited penetration of the polymer through the formation, and reduced the effectiveness of the polymer in reducing formation permeability.